Well they last rewrote the collections API in Scala 2.8 (I think), which was in 2010. If they redo it in 2.13 that'll have been about 8 years. That's certainly not C++ levels of backwards compatibility, but it's hardly frequent.

They do make some other breaking changes in 2.xx releases (which come out about every 1.5 - 2 years!), but I wouldn't really call them point releases, given that the 2.x hasn't changed in over a decade - that's like saying Python shouldn't make breaking changes in Python 2.6 -> 2.7. They don't generally make breaking changes in actual point releases (2.XX.yy).

Also, they're building an automated conversion tool (https://www.scala-lang.org/blog/2016/10/24/scalafix.html) for Dotty. As I said, compare this to Python 3. The Scala -> Dotty rewriter should be able to be more complete than 2to3 was however, mostly because they're not fixing many ambiguities like 2to3 was with encoding. Their rewriter is also based on a full sophisticated framework that can parse or unparse multiple versions of Scala, including Dotty, in one build.

Hopefully being able to rewrite a much, much higher percentage of code and the backporting of changes into Scala 2.13+ will make Dotty adoption happen faster (than Python 3) when it comes.